OLEDs are light-emitting diodes (LED) that emit with an emissive electro-luminescent layer composed of a film comprising an organic compound. The organic compound emits light in response to an electro-current stimuli running across the film. OLEDs can be made from small molecules or polymer sources. One of the advantages of an OLED display over other display formats is that OLED displays produce a lighted display without the need for a backlight. This allows for the production of deeper black levels of luminance on a thinner and lighter display screen than a corresponding liquid crystal display (LCD) screen. These deeper black levels allow for a higher contrast ratio on an OLED screen than a corresponding LCD screen in low ambient light conditions.
An OLED display 30 is shown in FIG. 1. An exemplary OLED display 30 consists of several parts including, a substrate 10, an anode 12, a plurality of organic layers 14, at least one conducting layer 16, at least one emissive layer 18, and a cathode 20. The substrate 10 may be plastic or glass that supports the other layers. The anode 12 removes electrons when a current is run through the device, whereas the cathode 20 injects electrons into the OLED display 30 when a current flows through the device. The organic layers 14 may be made of organic molecules or polymers depending on the type of OLED and are frequently deposited by vacuum deposition or vacuum thermalization or organic vapor phase deposition. However, inkjet printing can be used for depositing OLEDs onto the substrate 10. In a typical OLED, such as OLED display 30 the cathode 20 is stacked on top of the emissive layer 18 which is stacked on top of the conductive layer 16 which is stacked on top of the anode 12 which is stacked on top of the substrate 10.
The benefits of OLED displays over LCD displays are known. OLED displays are lighter weight than their LCD counterparts, can provide greater flexibility in the display, can have a wider viewing angle and a faster response time than corresponding LCD displays. Additionally, as described above, OLED displays are preferred in low-light conditions as OLED displays have a higher contrast ratio than their corresponding LCD displays. Additionally, OLEDs do not require a backlight which provides the thinner and lighter display than a corresponding LCD. At its most basic, an OLED display comprises a single organic layer between the anode and cathode. However, an OLED display having multiple layers of organic material is another possibility. Further, one of the most common OLED display configurations is a bilayer OLED comprising a conductive and emissive layer as described above.
OLED displays can be created using small molecules or polymers. Additionally, they can be created using a passive matrix (PMOLED) or an active matrix (AMOLED) addressing scheme. Small molecule based OLEDs are frequently created using vacuum deposition whereas polymer LEDs are frequently created using spin coating or ink jet printing. Additionally, while OLEDs have been described with the cathode on top of the stacking structure, inverted OLEDs, which provide the anode on the top of the stacking structure, are also known.
Transparent OLEDs are also known. Transparent OLEDs comprise transparent or semi-transparent contacts on both sides of an OLED device. These transparent or semi-transparent contacts allow displays to be made to be either top or bottom emitting. Top emitting OLEDs can have greatly improved contrast making it easier to view displays in direct sunlight.